Two classes of peptides are liberated by proteinases from fibrinogen: a) low molecular weight (LMW)-peptides, and b) high molecular weight (HMW)-peptides. Methods for the separation of these two classes have been worked out and the peptide fractions were characterized with respect to their amino acid compositions, peptide maps of the LMW-class, and optical properties related to secondary structure. The similarities and differences between the the peptides liberated by plasmin or trypsin from human or bovine fibrinogen were established. Quantitative estimates were made of the peptide yields with respect to the native molecule and its was found that in all cases the segments removed by the enzumes are similar and seem to originate from the same regions of the molecule.